Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various layers using lithography to form circuit components and elements thereon.
A transistor is an element that is utilized extensively in semiconductor devices. There may be millions of transistors on a single integrated circuit (IC), for example. A common type of transistor used in semiconductor device fabrication is a metal oxide semiconductor field effect transistor (MOSFET). Conventional MOSFETs have one gate electrode that controls a channel region, and are often referred to as single gate transistors. Early MOSFET processes used one type of doping to create single transistors that comprised either positive or negative channel transistors. Other more recent designs, referred to as complementary MOS (CMOS) devices, use both positive and negative channel devices, e.g., a positive channel metal oxide semiconductor (PMOS) transistor and a negative channel metal oxide semiconductor (NMOS) transistor, in complementary configurations.
Conventional bulk single-gate planar MOSFET devices are typically formed using a three-dimensional doping profile, which includes channel implantation, source and drain region implantation, lightly doped drain (LDD) extension implantation, and pocket/halo implantation processes. Scaling down bulk devices in size has limitations, because of an increase in dopant fluctuations and stronger parasitic short channel effects, due to lack of potential control in the channel region and the deep substrate. Therefore, transistors with multiple gates are under development, such as vertical multiple-gate fin field effect transistors (FinFETs) or tri-gate devices.
A double gate transistor has two parallel gates that face each other and control the same channel region. A FinFET is a vertical double gate device, wherein the channel comprises a vertical fin comprising a semiconductor material, typically formed on a silicon-on-insulator (SOI) substrate. The two gates of a FinFET are formed on opposing sidewalls of the vertical fin with an insulating material or hard mask formed on the top of the fin.
FinFETs may be used to form CMOS devices. One or more FinFETs can be used as a PMOS and/or NMOS transistor: often, two or more fins in parallel are used to form a single PMOS or NMOS transistor, for example. FinFETs can be scaled more aggressively than planar transistor structures, and show lower gate-induced drain leakage (GIDL) current than planar transistors. However, multiple gate transistors such as FinFETs are more difficult and complicated to manufacture than planar CMOS devices, and they require distinctly different materials and introduce a variety of processing challenges.
Furthermore, it is important to design CMOS devices so that a symmetric threshold voltage (Vt) for the NMOS and PMOS transistors of the CMOS device is achieved. However, it is difficult to find materials, device structures, and manufacturing processes that achieve a symmetric threshold voltage Vt as devices are made smaller, and more particularly for advanced transistor designs having multiple gates.
Thus, what are needed in the art are improved structures and manufacturing processes for multiple gate transistors.